be initiated by a larger-than-normal stimulus.
It coincides with the period during which
the voltage-gated K+ are open after voltage-
gated sodium channels have returned to their
resting state (hyperpolarizing phase). During
hyperpolarizing phase, continued K+ efflux
would tend to oppose any depolarization
caused by opening of voltage-gated sodium
channels. This is why a larger-than-normal
stimulus is needed to trigger another action
potential.
The absolute refractory period is like a
toilet after you have flushed and there is no
water in the flush tank: Even if you push
the handle, there’s no discharge. Once the
tank fills to half, you can flush, but it is too
weak. This is like the relative refractory
period. Once the tank is totally full you
can get a good, effective flush once again.
This is like the resting state.Neuron as simple RC circuit
The electrical properties of neurons can be
described in terms of electrical circuits.
Resistors:
A resistor is a component of a circuit that
resists the flow of electrical current. As
passive transporter, each ion channel can be
represented as a resistor in an RC circuit.
The more open channels in the membrane
represent more resistors in parallel (so an
overall smaller resistance).
Conductance (G) is the ease at which a
particle can move through a medium. It is
the reciprocal of resistance (R). Conductance
increases whenever more ion channels open
(e.g. due to voltage changes or binding of a
neurotransmitter to a receptor of ligand-gated
ion channel).
Capacitors
A capacitor is made up two conductor plates
separated by an insulator (dielectric). It can
store and separate a charge. In the neuron, the
conducting plates are the intracellular
and extracellular fluids, separated by the
non-conducting lipid bilayer of the plasma
membrane.RC Equivalent Circuit of a Neuron
Cell membranes that contain ion channels
can be modeled as an RC circuit. using a
battery, resistor and a capacitor.
The battery represents the stored potential
(EK, ENa) that is maintained across the cell
membrane (sodium-potassium pump).
Capacitor represents the intracellular and
extracellular solutions, separated by the
non-conducting membrane. The resistors
represents the ion channels. Variable resistors
represent voltage-gated ion channels as their
resistance varies with voltage.In a neuron, an action potential is triggered
when more ion channels are opened
(resistance is decreased and conductance is
increased). This causes rapid flow of ions
(current) leading to changes the potential
across the cell.